Despite the wide availability of dynamic mechanical analysis (DMA) results on polymers and composites, such data have rarely been applied to design of structures and components because the frequency-domain results obtained through this method are not directly applicable to most engineering problems. For thermosets, DMA is principally used to find maximum use and glass transition temperatures (Tg), which can determine the suitability of the material for application in a particular environment. However, for thermoplastics which are used above Tg, such as high density polyethylene (HDPE), and whose mechanical response is highly time-dependent (e.g., having an elastic modulus showing a change within quasi-static deformation rates of 0.0001-0.1/s), only having transition temperature information is not enough.
Measurement of properties at widely varying strain rates is often complicated by the limited speed ranges attainable within one testing setup or by a particular method. In addition, very low strain rate tests are time consuming and expensive to conduct, making it difficult to test multiple material samples at multiple strain rates and temperatures to develop a comprehensive understanding of mechanical properties of the material. Augmenting these present limitations, it is also noted that the correlation between results obtained from tensile or compressive tests with DMA results have not been established to develop a comprehensive understanding of the time and temperature dependent behavior of materials.
DMA provides storage modulus E′ and loss modulus E″ data. However, these two parameters are not usually parameters in engineering design. Instead, most engineering designs utilize Young's modulus (also known as the elastic modulus). Young's modulus is a measure of elasticity equal to the ratio of the stress acting on a material to the strain produced. In order to determine the Young's modulus, tensile and compression tests are conducted at very slow deformation rates (10−6 to 10 s−1) using universal test machines. Tensile and compression tests are conducted at high strain rates (500 to 5000 s−1) using split-Hopkinson pressure bar. Although Young's modulus can be calculated by this method at a high strain rate, the measurements are often not very reliable and the method is complicated. In addition, tensile and compression tests are done at various temperatures to obtain a full data set that describes material behavior over a wide range of strain rates and temperatures. High and low temperature split-Hopkinson pressure bar experiments are very complicated because temperature dependent correction factors are required for the wave speed and modulus of the bar material used in equipment, among other additional parameters needed to conduct calculations. Much of this information is not readily available in literature and needs additional experimentation. Measurement of properties at widely varying strain rates is often complicated by the limited speed ranges attainable within one testing setup or by a particular method. In addition, very low strain rate tests are time consuming and expensive to conduct, making it difficult to test multiple material samples at multiple strain rates and temperatures to develop a comprehensive understanding of mechanical properties of the material. Augmenting these present limitations, it is also noted that the correlation between results obtained from tensile or compressive tests with DMA results have not been established to develop a comprehensive understanding of the time and temperature dependent behavior of materials.
DMA is considered the most sensitive method to locate thermal transitions including those in crystallization and resin curing. When combined with other spectroscopy methods, information from DMA can reveal activation of different modes of motion of the polymer chains. DMA is also used to gain information on temperature sensitivity of the behavior of polymer blends, pharmaceutical and biomedical materials, and micro- and nano-composites. DMA data provides storage modulus E′, loss modulus E″, damping parameter tan δ, and glass transition temperatures Tg. However, the relation of the storage modulus E′ and the loss modulus E″ to Young's modulus (elastic modulus) at different strain rates has not been developed, which has been a major limitation in using DMA results in mechanical design.
A need exists for improved technology for transforming frequency-domain DMA data into a time-domain representation which can yield more readily useful information about the material behavior.